This invention relates generally to electromagnetic flowmeters, and more particularly to improved means for eliminating in phase noise components from the signal generated by a flowmeter.
In an electromagnetic flowmeter, the fluid whose flow rate is to be metered is conducted through a flow tube provided with a pair of diametrically-opposed detecting electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the fluid passing through the tube intersects this magnetic field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the fluid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
In a conventional electromagnetic flowmeter, even when the fluid to be measured is not flowing through the tube, in phase noise is developed across the detecting electrodes. This in phase noise varies with time, thereby producing a changing zero drift. Thus while the conventional flowmeter has good linearity characteristics in flow measurement, it suffers from the drawback that flowing liquids of low velocity cannot be precisely measured.
There are two main sources for in phase noise. First is in phase noise arising from the surface condition of the electrodes, and second is noise due to the condition of the magnetic field. Inasmuch as in phase noise resulting from the surface condition of the electrodes can be obviated by expedients disclosed in our Japanese Utility Model 917,091, the present invention is concerned solely with in phase noise resulting from the second source, namely the condition of the magnetic field.
The causes of in phase noise arising from the condition of the magnetic field fall into two distinct classes. The first class covers in phase noise resulting from an asymmetry in the main magnetic fluxes passing through respective regions separated by a plane that is parallel to the lines of flux and including the electrodes. The second class covers in phase noise induced in a loop constituted by the lead wires to the electrodes in combination with the liquid bridging the electrodes, by reason of the asymmetry of leakage magnetic fluxes arising from the asymmetry of the main magnetic fluxes.
Means have heretofore been proposed by us to eliminate in phase noises whose causes fall into the above-described classes. Thus our Japanese Pat. No. 616,372 discloses means to overcome in phase noises whose cause lies in the first class, while Japanese patent application Ser. No. 60,375/1974 covers means applicable to in phase noise whose cause lies in the second class.
The disadvantages of the arrangements disclosed in our prior Japanese patent and patent application, identified above, is that the respective compensating means must be independent of each other and disposed in the narrow space between the magnetic core surrounding the excitation coil of the flowmeter and the flow tube.